This invention relates to a photocathode for pickup tubes and for image intensifier tubes.
It is a known practice to construct a photocathode having the following main components:
a so-called window layer consisting of P.sup.+ type semi-conductor material in which the forbidden band is of sufficient width to ensure that said layer is transparent to the wavelengths of the light to be detected and which is bonded to a glass wall for receiving the light to be detected;
a so-called absorption layer consisting of P.sup.+ type semiconductor material in which the forbidden band is of sufficiently small width to convert the photons of the light to be detected into electron-hole pairs;
a so-called emission layer consisting of material which produces negative electron affinity at the end of the absorption layer in order to emit into the vacuum the electrons which are liberated within the absorption layer.
The maximum detectable wavelength is limited by the width of the forbidden band of the material which constitutes the absorption layer. By applying a positive bias to that end of said layer which is opposite to the window layer, it is possible to employ materials which have a small forbidden bandwidth while maintaining good emission efficiency and it is therefore possible to detect light having longer wavelengths.
A bias can be applied to the absorption layer by means of a connection with said layer or by a very thin metallic electrode interposed between said layer and the emission layer. A photocathode of this type is described in the article "Photoelectric Imaging in the 0.9-1.6 Micron Range" by J. J. Escher et al., IEEE-EDL2, 123-125 (1981).
In order to construct a pickup tube for use at very low light levels and especially in order to construct an image-intensifier tube, a known practice consists in placing a microchannel plate downstream of the photocathode. The microchannels are supplied by a high-voltage generator and permit multiplication of the electrons emitted from the photocathode into the vacuum. A microchannel plate produces electron multiplication with a high degree of efficiency but imposes many technological constraints including in particular the use of a high-voltage generator. The aim of the invention is to produce an internal-amplification photocathode which permits the use of a microchannel plate having a lower gain, thus reducing the need for technological constraints and even dispensing with the need for a microchannel plate. The object of the invention is a photocathode provided with an absorption layer having a particular structure which produces multiplication of electrons while avoiding any appreciable multiplication of the hole current since this latter gives rise to a dark current which constitutes noise.